Field of the Invention
The present invention relates to an actuator, sample positioning device, and charged particle beam system.
Description of Related Art
A device (known as a positioning goniometer) for placing an electron microscope sample in position is used such that the sample is brought into a desired position and held there. Usually, an actuator having a magnetic motor is used as such a positioning device for moving the sample in the X- and Y-axis directions.
An actuator of this type is disclosed, for example, in JP-A-6-300106 and has a composite shaft consisting of a threaded shaft provided with a ball spline groove that is formed in the outer surface of the threaded shaft so as to extend axially of the threaded shaft. The threaded shaft is formed with a ball threaded groove. An outer cylinder for a spline, a ball screw nut, and a hollow motor are incorporated in this order in the composite shaft from the load application side of this composite shaft.
With the actuator of JP-A-6-300106, if the hollow motor is rotationally driven, the rotational driving force of the motor drive shaft is transformed into an axial thrust force of the composite shaft via the ball screw nut. An axial reaction force responsive to this thrust force is borne by the bearings in the hollow motor with the consequent linear motion of the composite shaft. The axial load acting on the composite shaft is borne by the ball nut screw, while the torque is borne by the outer cylinder for a spline. Radial loads and moment loads in all directions are borne by the outer cylinder for a spline and by the ball screw nut. Attachments such as fingers mounted to an attachment mounting portion are placed in position along the X-, Y-, and Z-axes and along moments Mx, My, and Mz around the axes.
In the actuator set forth in JP-A-6-300106, the outer cylinder for a spline is cylindrical in shape and has an inner surface provided with a ball-rolling groove corresponding to the ball spline groove formed in the composite shaft. Multiple balls that bear the load are rollably mounted between the ball spline groove and the ball-rolling groove. A ball holder is mounted on the inner surface of the outer cylinder for a ball spline to hold the balls so as to move cyclically in alignment. The holder is provided with a plurality of cyclic ball paths to permit the balls to move cyclically between a loaded region and an unloaded region existing between the outer surface of the holder and the inner surface of the outer cylinder for a spline.
In the actuator of JP-A-6-300106, the balls are preloaded when they are between the ball spline groove formed in the composite shaft and the ball-rolling groove formed in the outer cylinder for a spline. However, the balls are not preloaded when they are in the cyclic ball path formed in the ball holder. Therefore, when the balls move into the cyclic ball path from between the spline groove and the ball-rolling groove, the balls are varied from the preloaded state to the non-preloaded state. At this time, an impact acts on the actuator, vibrating the composite shaft. Furthermore, when the balls move from the cyclic ball path to between the spline groove and the ball-rolling groove, the balls are varied from the non-preloaded state to the preloaded state. Consequently, the composite shaft vibrates similarly.
If the composite shaft, i.e., the output shaft of the actuator, vibrates, the driven object is also vibrated. For example, if the output shaft of an actuator of a device for placing an electron microscope sample in position vibrates, when the sample is moved in a given direction, vibration of the sample is caused as well as the motion of the sample in the given direction. Therefore, if a user attempts to move a sample while checking the field of view of the sample, for example, this is difficult to perform because the sample is vibrated, i.e., the field of view is vibrated.